Problem 9 · 2016 Math Kangaroo
Medium
Arithmetic & Operations
divisioncareful-counting
A house has 12 rooms. Each room has two windows and one light. Only when the light is on in a room are both of its windows lit. Yesterday evening, 18 windows were lit. In how many of the rooms was the light off?
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Answer: B — 3
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Hint 1 of 2
A room shows light in both its windows only when its light is on.
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Hint 2 of 2
Find how many rooms were lit, then compare with the 12 rooms total.
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Approach: windows come in pairs per lit room
- Each lit room shows 2 illuminated windows, so 18 windows means 18 ÷ 2 = 9 rooms were lit.
- There are 12 rooms in all, so 12 − 9 = 3 rooms had the light off.
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